1. The sunlight readable one is more expensive because the touchscreen is optically bonded to monitor, thus allowing more light from the monitor pass through and removing most of the glare. The high brightness lilliput uses the same touchscreen as all the other lilliputs, it simply has a backlight which is brighter in nits than the standard lilliput. Whether or not you should use the sunlight readable lilli depends on personal preference and screen placement (per vehicle). Knowing the dash on your mustang and knowing that the DIN is technically facing the rear window rather than the driver, i'd buy the sunlight readable model (budget allowing of course)
2. Sunlight Readable somewhat defeats the purpose of needing a high brightness backlight. The high brightnes models were released to somewhat circumvent the glare problem, it definitely is a band-aid fix.
3. The metra kit will fit your DIN but will not fit the screen area of your touch screen. You will probably need both the Metra kit (to fill the gaps left from the factory stereo in the dash) AND the bbx-1 (to make the screen cleanly mate into the metra kit). The H-Backet should be included with frame purchase regardless if you buy preassembled or do it yourself, provided you order the BBX-1 that matches your lilliput model